Have a 2007 21" imac. friend installed 2, 4GB memory for me, Apple genius says that's what causing slow downs on my machine? when they tested it only had a 4GB & a 2GB cards, so someone along the way did a switch on me. Apple says my machine can only handle 4GB total. i wonder if that's true? seems like it ran quite well for a long time, & it used to corroborate that it had 8 GB memory, so something fishy has occurred. machine is running quirky & would like to have max memory. Would appreciate some knowledgeable input...
Crucial, UK state that 4GB is the maximum amount of RAM that your iMac officially supports.
If you put more in and it worked then this could be seen as a 'bug', and wouldn't be supported by Apple. Such abilities could also be revoked through software/firm was updates.
according to Mactracker the mid 2007 imac 20inch has: Maximum Memory 6.0 GB (Actual) 4.0 GB (Apple) Memory Slots 2 - 200-pin PC2-5300 (667MHz) DDR2 SO-DIMM
you should also make sure that you always have the same amount on both slots in your case two 3gb however 3gb rams are rare therefore more expensive
In another forum some users have confirmed 2007 iMac works with 8 GB with no issue after the el Captain upgrade.